This invention relates to a differential pressure transmitter and more particularly to a differential pressure transmitter which can properly compensate for errors caused by high static pressure.
As described in the Japanese Patent Application Laid-Open No. 120142/1983, No. 61637/1985 and No. 56465/1986, and the Japanese Utility Model Application Laid-Open No. 167432/1983, conventional pressure differential transmitters detect a static pressure by utilizing a differential pressure detecting diaphragm and a semiconductor piezoresistive gauge provided on the same chip on which the diaphragm is mounted. With this construction, however, there is a high level of crosstalk between the differential pressure detecting gauge and the static pressure detecting gauge, which makes it difficult to compensate for errors caused by high static pressures. This is a major obstacle in the way to improving the accuracy of the differential pressure transmitter.
U.S. Pat. No 4,528,855 discloses a differential pressure transmitter in which a static pressure applied to the upper and lower surfaces of an annular diaphragm surrounding the differential pressure sensor is used to deform the differential pressure sensor and thereby obtain changes in resistance of a plurality of semiconductor piezoresistive gauges.
There are many kinds of differential pressure sensors that detect a differential pressure and a temperature. FIG. 1 is a vertical cross section showing one representative construction of the differential pressure transmitter currently in use, which is introduced in the Japanese Patent Application Laid-Open No. 61637/1985. A silicon diaphragm 1 has on a thin portion lA a semiconductor piezoresistive gauge group 2 that senses a differential pressure .DELTA.p and, on a fixed thick portion lB, a semiconductor piezoresistive gauge 3 that senses a temperature. Both of these gauges are diffused into the silicon diaphragm 1. These piezoresistive gauges are formed so that they will not respond to a high static pressure P, and each of the gauges is connected to external circuits through lead wires 6 that are extracted from an airtight hermetic sealed terminal 5. Hence, the semiconductor piezoresistive gauge group 2 generates a signal proportional to a differential pressure .DELTA.p and the semiconductor piezoresistive gauge 3 produces a signal proportional to a temperature T. The external circuit will then produce a differential pressure signal without errors caused by temperature variations.
A static pressure P applied on both sides of the silicon diaphragm 1 of the differential pressure transmitter is normally as high as 100 atmospheres. At such a high static pressure, any imbalance in liquid contraction between chambers 7A, 7B on both sides filled with liquid or any deformation of a cage 4 will cause deformation in the silicon diaphragm 1. This in turn results in a change in resistance of the semiconductor piezoresistive gauge group 2. Hence, the signal representing a differential pressure is superimposed with a signal produced by a static pressure, making it impossible to output a correct differential pressure signal. In other words, the signal output from the differential pressure transmitter is affected by the static pressure and contains errors. To prevent errors from being caused by the static pressure requires that the amounts of liquid sealed in the chambers 7A, 7B be strictly equal to each other and that the cage 4 be given a greater rigidity so that it will not be deformed by the static pressure P. These requirements constitute a large constraint in design and manufacture, inhibiting further reduction in size and cost of the differential pressure transmitter.